Radiological Training for Accelerator Facilities
Large linear accelerators are of two types:
the drift tube and
the wave guide.
N Drift tube accelerator: In the drift-tube accelerator, the particle is accelerated through gaps
between sections of a hollow conducting cylinder. When the particle crosses a gap, it is
accelerated and the voltage oscillates so that when the particle reaches the next gap, it is
accelerated again. Since the frequency of the voltage is kept constant, sections of the tube
must be of different lengths as the particle accelerates.
N Wave guide accelerator: The wave guide accelerator, invented in 1947 by D.W. Fry, uses
long wavelength electromagnetic waves guided through a hollow conductor. Since electrons
of moderate (about 2 MeV) energy already travel with nearly the speed of light, they can keep
up with the wave and thus ride the force of the electric-vector part of the wave somewhat as a
surfer can ride an ocean wave.
Lawrence and the Development of the Cyclotron
The attempt to reach higher energies than were available with the devices of the early days led
Ernest O. Lawrence of the University of California to propose the first cyclotron. The idea was
simple. Instead to trying to accelerate the particles in one pass through, it was proposed that
many small accelerations be used so that their sum would be what was desired. The idea was to
use a magnetic field to make the charged ion move in a circular path.
Lawrence succeeded in substituting a circular race track for the straight-line track of the linear
accelerator. Each time the particle went around, it could be accelerated a little bit thus attaining
energies above that used in previous accelerators and thus with comparatively low voltages. In
describing the first cyclotron, Lawrence and Livingston wrote in 1932 that "experimental
difficulties go up rapidly with increasing voltage." This first device, barely 1 foot across,
produced 1,200,000-volt (1.2 MeV) protons.